1. Field of the Invention
The present invention relates to an exhaust gas purification method for an internal combustion engine using an NOx absorption catalyst, and more particularly to an exhaust gas purification method for an internal combustion engine for preventing deterioration of exhaust gas, and achieving good fuel consumption while sufficiently securing lean operation time.
2. Description of the Related Art
Generally, in operation control of an internal combustion engine, improvement of fuel consumption is an objective and an air/fuel ratio is controlled to maintain a lean ratio. In order to reduce NOx that is released, an NOx absorption catalyst is used.
Further, in the NOx absorption catalyst, an absorption function (NOx reducing function) suffers when an NOx absorption amount exceeds an amount permissible by the NOx absorption catalyst. Therefore, in order to maintain the absorption function, it is necessary to periodically release and reduce the NOx that has been absorbed.
Therefore, conventionally, in an exhaust gas purification system using an NOx absorption catalyst, the NOx released during the lean operation state (over-oxygenated state) of the internal combustion engine is absorbed into the NOx absorption catalyst, and at every predetermined cycle, the operation state of the internal combustion engine is switched to a rich operation state, and the NOx absorbed in the NOx absorption catalyst is released and the NOx is simultaneously reduced.
This type of exhaust gas purification method for an internal combustion engine can be referenced in JP 2600492 B, for example.
In this way, in the exhaust gas purification method using the NOx absorption catalyst, in order to release the NOx form the NOx absorption catalyst to reduce the NOx, it is important to appropriately control the timing of the switching from the lean operation to the rich operation.
In the above method disclosed in JP 2586739 B, etc., for example, an NOx amount already absorbed in the NOx absorption catalyst is estimated, and in a case where it is determined that the estimated absorption NOx amount is greater than a predetermined permitted amount, the operation state of the internal combustion engine is switched from lean to rich.
FIG. 6 is an explanatory diagram showing an exhaust gas purification states according to the conventional method. The diagram shows released NOx amounts according to the amount of an inflow NOx amount flowing into the NOx absorption catalyst.
In FIG. 6, a horizontal axis represents time, a vertical axis represents the NOx amount (inflow amount and emission amount), a diagonal line region represents the NOx amount absorbed in the NOx absorption catalyst, and a white arrow represents the NOx amount that can be processed by the NOx absorption catalyst. Cases where the inflow NOx amount (see the broken line) is great (top), and is small (bottom), are shown for comparison.
As is clear from FIG. 6, even when the estimated absorption NOx amount (see the diagonal line portion), if the inflow NOx amount (see broken line) is great, a portion of the inflow NOx amount flows out (is released) to a downstream of the NOx absorption catalyst. By contrast, in the case where the inflow NOx amount is small, the NOx absorption capability (see the white arrow) still has a margin, so it is understood that the lean operation may be continued.
This shows that even when the estimated absorption NOx amount is the same as the NOx amount processable by the absorption catalyst, if the inflow NOx amount is small, the entire NOx amount is processed and does not leak downstream. By contrast, in the case where the inflow NOx amount is great, the inflow NOx amount cannot be completely processed, and thus flows out downstream.
In other words, according to the method such as the conventional method, in which the operation state is switched from lean to rich in the case where it is determined that the NOx amount inside the NOx absorption catalyst exceeds the permitted amount, when the inflow NOx amount entering the NOx absorption catalyst changes, it is difficult to switch the operation state from lean to rich at appropriate timing, and this can adversely instigate exhaust gas deterioration or decrease the benefits of fuel consumption and the like.
As described above, in the conventional internal combustion engine exhaust gas purification method, the operating state was switched from lean to rich based only on the estimated/calculated absorption NOx amount, without considering the inflow NOx amount going into the NOx absorption catalyst. Therefore, the operating state can not be switched from lean to rich at appropriate timing, and thus there was a problem of causing exhaust gas deterioration, and reduction of benefits of fuel consumption.